Transmit and receive isolation for ultrasound scanning and methods of use

ABSTRACT

Methods and systems for isolating transmit and receive circuitry at an ultrasound transducer element are provided. Separate electrodes or electrodes on opposite sides of a transducer element are connected to the separate transmit and receive paths or channels. Instead of high voltage transmit and receive switching, the transducer element isolates the transmit channel from the receive channel. The transmit channel includes circuitry for limiting the voltage at one electrode during receive processing, such as a switch operable to connect the electrode to ground. The receive channel includes circuitry for limiting the voltage at an electrode during transmit processing, such as a diode clamp preventing voltage swings greater than diode voltage at the electrode. Limiting the voltage provides virtual grounding or a direct current for either of the transmit or receive operation. Using a transmit channel discussed above or other transmit channels, a unipolar pulse may be generated starting at one voltage and ending at a different voltage. For example, a unipolar pulse is generated starting at a zero voltage value and ending on a positive voltage value. A subsequent unipolar pulse is transmitted by starting at the positive voltage value and ending on the zero voltage value. These mirrored unipolar transmit waveforms may be used for phase inversion imaging, such as adding responsive received signals for isolating harmonic information.

BACKGROUND

The present invention relates to receive circuits for ultrasoundimaging. In particular, receive circuits for use with differenttransducers are provided.

Ultrasound imaging for echocardiography applications requirestransducers with high volume-per-second rates for scanning. Forreal-time imaging of moving structures, 20 or more, such as 35, two orthree-dimensional representations are generated each second. Largeamounts of information are communicated from an ultrasound probe to anultrasound system base unit.

Various transducers and associated beamformers have been provided forthree-dimensional ultrasound imaging. Currently, mostly mechanicaltransducers are used. However, the associated imaging is not provided inreal time and typically requires ECG gating. Two-dimensional transducerarrays for faster electronic/electronic steering and volume acquisitionalso have been provided. For example, sparse two-dimensional arrays orfully sampled two-dimensional arrays have been used. Sparse arraysprovide poor contrast resolution.

Fully sampled two-dimensional arrays use expensive additionalbeamforming hardware. Two-dimensional arrays repetitively generatetransmit beams and responsive receive beams. The beams areelectronically steered within the three-dimensional volume. Electronicsteering requires a system channel for each of the elements used. Sincethe number of elements in a two-dimensional array is high, the number ofchannels required is high. More channels require a greater number ofcables. Providing beamforming or partial beamforming within the probe ofthe transducer array may reduce the number of cables required, but therequired number of channels and hardware for sampling thetwo-dimensional array is still high. Furthermore, analog delays used forbeamforming in the probe are expensive and large, and the beamformer inthe probe may have limited programmability.

Transducer arrays include elements with a ground electrode and a signalelectrode switchably connected to separate transmit and receive systemchannels. With beamforming capabilities built into the probe, highvoltage transistors or diodes operating as switches to isolate thetransmit channels from the receive channels are also included within theprobe. These high voltage devices are not easily integrated with thebeamforming circuitry, so require additional space.

In one system disclosed in U.S. Pat. No. 5,622,177, the number of systemchannels and cables is reduced by using time division multiplexing. Datafrom a plurality of elements is multiplexed onto one signal line.However, time division multiplexed data has different characteristicsthan conventional data representing the signal from a single transducerelement. Receive circuitry designed for use with conventional data mayimproperly introduce noise or errors in time division multiplexed data.

BRIEF SUMMARY

By way of introduction, the preferred embodiments described belowinclude methods and systems for isolating transmit and receive circuitryat an ultrasound transducer element. Separate electrodes or electrodeson opposite sides of a transducer element are connected to the separatetransmit and receive paths or channels. Instead of high voltage transmitand receive switching, the transducer element isolates the transmitchannel from the receive channel. The transmit channel includescircuitry for limiting the voltage at one electrode during receiveprocessing, such as a switch operable to connect the electrode toground. The receive channel includes circuitry for limiting the voltageat an electrode during transmit processing, such as a diode clamppreventing voltage swings greater than diode voltage at the electrode.Limiting the voltage provides virtual grounding or a direct current foreither of the transmit or receive operation.

Using a transmit channel discussed above or other transmit channels, aunipolar pulse may be generated starting at one voltage and ending at adifferent voltage. For example, a unipolar pulse is generated startingat a zero voltage value and ending on a positive voltage value. Asubsequent unipolar pulse is transmitted by starting at the positivevoltage value and ending on the zero voltage value. These mirroredunipolar transmit waveforms may be used for phase inversion imaging,such as adding responsive received signals for isolating nonlinearresponse information.

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Furtheraspects and advantages of the invention are discussed below inconjunction with the preferred embodiments.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The components and figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a block diagram of one embodiment of an ultrasound system forreceiving different types of signals from different transducer probes.

FIG. 2 is a flow chart diagram of one embodiment of a method forreceiving data associated with a plurality of transducer elements on asingle cable.

FIG. 3 is a block diagram of one embodiment of a transducer withisolated transmit and receive channels.

FIG. 4 is a circuit diagram of one embodiment of a transmitter.

FIG. 5 is a circuit diagram of an alternative embodiment of atransmitter.

FIG. 6 is a flow chart diagram of one embodiment representing use of theisolated transmit and receive channels of FIG. 5 to transmit and receiveacoustic information.

FIG. 7 is a graphical representation of unipolar pulses with oppositephases.

FIG. 8 is a graphical representation of a multi-dimensional transducerarray.

FIG. 9 is a perspective view of one embodiment of an interior of a probeincluding a multi-dimensional transducer array connected with circuitboards.

FIG. 10 is a cross sectional diagram of one embodiment of amulti-dimensional array assembled from modules.

FIGS. 11A and 11B are graphical representations of steps performed formanufacturing a multi-dimensional array using pre-diced modules.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Faster or more complex two-dimensional and three-dimensional ultrasoundimaging is provided by using multiplexing. A multiplexer is providedwithin a probe so that information from multiple transducer elements aremultiplexed onto one signal channel for transmission to a base unit orultrasound system for further processing. To avoid having differentsystems for different types of transducers, receive circuitry of anultrasound system is operable in different modes based on the format ofsignals provided by the transducer. To further minimize the number ofchannels connecting a probe to an ultrasound system without adverselyaffecting the size of the probe, a transmit channel is separated fromthe receive channel by a transducer element. This separation isolatesthe transmit channel while minimizing integration of high voltagedevices within the probe. To allow the element to isolate the transmitand receive channels, the transducer array is manufactured fromseparately diced modules, each module including signal traces toopposite sides of each element.

The developments discussed above for multiplexing may be usedindependent of the multiplexing or other features. These independentdevelopments or features are described in three general sections below.Receive circuitry for receiving information associated with differentsignal formats or for receiving just multiplexed format is describedfirst. Isolation of the transmit path from the receive path using atransducer element and associated methods of use are described second.Finally, transducer arrays and methods of manufacture are described.

Receive Circuitry:

FIG. 1 shows a block diagram of one embodiment of an ultrasound system10. The system 10 includes a base unit 12 with receive circuitry 14 andan image processor 16. The receive circuitry 14 is operable to connectwith different types of transducer probes 18, 20 via a cable 22. Aplurality of receive circuits 14 are electrically connectable with theprobes 18, 20 for processing signals from an array of elements 24.Additional, different or fewer components may be provided in the system10, such as providing only one type of transducer probe 18, 20.

One transducer probe 20 comprises an array of piezoelectric ormicroelectromechanical elements 24 for transducing between acoustic andelectrical energies. The probe 20 includes a single element, a lineararray of elements or a multi-dimensional array of elements. The probe 20also includes a housing covering the array. The housing is shaped to bea hand-held device or may be shaped for insertion into cavities orcardiovascular system of a patient. The probe 20 connects to the receivecircuitry 14 using a cable 22 for each element 24 of the array. Eachcable 22 transmits an analog signal representing the acoustic energyreceived at a single element 24. The signaling provided on the cable 22from the probe 20 are conventional signals free of multiplexing or otherintermediate circuits between the element 24 and the connector 32. Theprobe 20 provides signals or other information formatted differentlythan the signals from probe 18.

The probe 18 includes a linear or multi-dimensional array of elements 24connected with a multiplexer 26. In one embodiment, 1,536 elements 24are configured as a two-dimensional or multi-dimensional array. Theprobe 18 also includes a housing covering the array. The housing isshaped to be a hand-held device or may be shaped for insertion intocavities or cardiovascular system of a patient. In one embodiment, thetransducer probe 18 comprises multi-dimensional transducer probemanufactured as discussed below using modules, but other linear ormulti-dimensional arrays using a ground plane or with separate signalingmade from one PZT component or modules may be used.

The probe 18 includes preamplifiers 35 and time gain controls 37 as areceive channel 64 prior to multiplexing. The receive channel 64connects with the element 24. The preamplified and time gain controlledinformation are provided to sample and hold circuits 60. The sample andhold circuit 60 comprise analog delays for multiplexing analoginformation from multiple elements 24 onto one output. In the preferredembodiment, no sample and hold function exists. Analog waveforms areinterleaved in time with no “hold” and no “analog delay” operation. Useof a sample and hold is not a requirement but it is one possiblealternative.

In one embodiment, the receive circuits in the probe 18 dissipate lessthan 5 watts. In one embodiment, one multiplexer 26 is provided forevery eight elements 24, but a single multiplexer may be provided forall elements or for a different number of elements. The multiplexer 26comprises an analog or digital network of switches responsive to a probecontrol 28. In one embodiment, the multiplexer 26 combines signals froma plurality of elements 24 using time division multiplexing. Inalternative embodiments, frequency multiplexing or other multiplexingschemes now known or later developed may be used. The probe control 28controls the multiplexer 26 in response to a clock signal so that analogsignals from each of the elements are assigned a specific time slotwithin a frame of time division multiplex information. In oneembodiment, the probe 18 and associated multiplexer 26 comprise the timedivision multiplexing probe discussed in U.S. Pat. No. 5,622,177, thedisclosure of which is incorporated herein by reference. Additional,different or fewer components may be provided in a probe 18, such asproviding additional amplifiers or filters in the probe 18 or a probefree of the preamplifiers or time gain controls.

The multiplexer 26 outputs time division multiplex or other formatteddata to a line driver 30. The line driver 30 comprises an amplifier orother device integrated with or separate from the multiplexer 26 fortransmitting the multiplexed information over the cable 22. Separatecables 22 may be provided for additional multiplexers 26, such as 192 or256 cables 22.

The base unit 12 comprises an ultrasound imaging system, such as ahand-held, cart based or other system for generating a two-dimensionalor three-dimensional representation of a patient. The receiver circuitry14 receives information from one or more transducer probes 18, 20 forbeamformation, detection and other ultrasound image processing by theimage processor 16.

The receive circuit 14 includes a connector 32, a mode control processor34, a preamplifier 36, a time gain control circuit 38, a low pass filter40, a buffer 42, an analog-to-digital converter 44, a digital equalizer46, a digital demultiplexer 48, an analysis processor 50, and aselectable delay 52. Additional, different or fewer components may beprovided. The receiver circuit 14 comprises one or various combinationsof two or more of the components described above. For example, thereceiver circuitry comprises just the preamplifier 36 or just the lowpass filter 40. The receive circuit 14 is operable with the transducerprobe 20 where the signals from elements 24 may or may not be amplifiedand/or processed before transmission to the base unit 12. A second modeof operation allows transmission of time division or other multiplexedsignals representing a group of elements along one signal line or cable22. The receive circuitry 14 comprises a single receive channel withinthe base unit 12. Multiple receive channels for association withdifferent cables 22 and different elements 24 are provided.

The connector 32 comprises a female or male latch with electricalcontacts for connecting with a bundle of cables 22. The connector 32 isoperable to connect with different transducer probes 18, 20. Forexample, a probe with time division multiplexing capabilities isconnected to the connector 32. As another example, the probe 18 isdisconnected from the connector 32 and the other probe 20 is connectedto the connector 32. The connector 32 releasably maintains physical andelectrical contact with the bundle of cables 22. In alternativeembodiments, a separate connector 32 is provided for different probes18, 20. The same base unit 12 and receive circuit 14 may be used forreceiving and processing information from different types of transducerprobes 18, 20. For example, the connector 32 connects with the probe 18for imaging using a fully populated two-dimensional or 1.5 dimensionalarray. Time division multiplexing allows for steering in two spatialdimensions for two-dimensional or three-dimensional imaging whileminimizing the number of cables 22 for communicating signals to the baseunit 12. The same connector 32 connects with the other transducer probe20 for ultrasound imaging using signals free of multiplexing. In oneembodiment, multiple connectors 32 are provided with relay orsolid-state switching into the common receive circuit 14 to providerapid access to a selection of transducers. Each individual connector 32may accept either multiplexed transducers 18 or conventional transducers20.

The mode control processor 34 comprises a control processor, generalprocessor, application specific integrated circuit or other analog ordigital device for controlling components of the receive circuit 14,such as the preamplifier 36 and low-pass filter 40. In response to aconfiguration entered by the user, in response to control signalsprovided by the probe control 28, in response to a detection by theconnector 32 of a type of probe, or in response to analysis of signalsreceived from the ultrasound probe 18, 20, the mode control processor 34configures one or more components of the receive circuitry 14 forprocessing in accordance with the type of data or data format providedby the probe 18, 20. The characteristics of the receive circuit areconfigured as a function of the data format.

The preamplifier 36 comprises transistors or other analog or digitaldevices for providing a low noise, wide band matched receiver. Thepreamplifier 36 is programmable or responsive to the mode controlprocessor 34 for programming characteristics of the preamplifier. Foroperation with the transducer probe 20 or operation with signalsrepresenting a single transducer element 24, the preamplifier 36 isprogrammed to have a impedance characteristic similar to or at theimpedance of the element 24 and the cable 22, such as 1 kOhm impedance.The impedance matches a generalization based on expected variations incable impedances for different types of probes 20. The preamplifier 36may alternatively be programmable for specifically matching specifictypes of probes 20 with different cables 22, cable lengths or elements22. Preamplifier input impedance, gain and frequency response may becontrolled either by selectable switched components or by alteringpreamplifier bias current. In practice, both methods may be employedsimultaneously within an integrated circuit realization. For operationwith multiplexed signals, the preamplifier 36 is programmed for animpedance match to the line driver 30 or other output circuitry of theprobe 18. For example, the preamplifier 36 is programmed to provide anapproximate 50 ohms impedance match. In alternative embodiments,different preamplifiers 36 are selected by the mode control processor.

In another embodiment, the gain characteristic of the preamplifier 36 isselected as a function of the format of signals or type of probe 18, 20.Multiplexed transducers 18 may require lower preamplifier gain thanconventional transducers 20 because signals are already preamplifiedwithin the transducer prior to multiplexing. Also, noise performance ofthe system preamplifier 36 is not as stringent for multiplexedtransducers 18 with integral preamplifiers 36, so a degraded noisepreamplifier might be desirable to save power or otherwise optimizeinput impedance, gain, and frequency response.

Another programmable characteristic is the bandwidth of the preamplifier36. For multiplexed information, the preamplifier 36 is not band limitedor operates over a wide band, such as passing frequencies having asymbol rate of more than twice the center frequency of the transducerarray (e.g., more than 5 MHz, 30 MHz or 100 MHz or more) for timedivision multiplexing. For information free of multiplexing, thebandwidth may be 2-15 MHz, such as associated with ultrasoundfrequencies or the frequency band of the transducer. Othercharacteristics of the preamplifier 36 may be adapted or altered as afunction of the data format provided from the transducer probe 18, 20.

Signal conditioning blocks may be included in the multiplexer 26 or withthe preamplifier 36 to provide pre- and post-equalization for frequencydependent losses in the cable 22. In alternative embodiments, thedigital equalizer 46 provides post-equalization. The equalization mayminimize inter-symbol interference. For example, pre-emphasis orhigh-frequency boost could be applied prior to driving the cable tocompensate for frequency-dependent cable losses. An all-pass phasecorrection filter could also be implemented in the system receiver 14 tofurther reduce inter-symbol interference prior to the ADC.

The time gain control 38 (i.e. depth gain control) comprises anadjustable gain amplifier for variably amplifying analog signals. Forsignals representing a single element 24, the variable gain comprises a40 to 80 dB range, but other gains may be used to account for theapproximately one dB per MHz per centimeter of depth attenuation ofultrasound signals. The time gain control 38 operates the same ordifferently for multiplexed signals. Where a time gain control 38 isprovided in the probe 18, the time gain control 38 of the receivecircuit 14 provides less or no variable gain for multiplexed signals.Where the time gain control 38 applies a variable gain, the applicationof the gain accounts for the time division multiplexing by applying asame gain within each frame of signals from multiple elements 24.

The low-pass filter 40 comprises an anti-aliasing filter implemented asa finite impulse response or infinite impulse response filter. Thelow-pass filter 40 band limits signals so signals greater than ½ thedigital sampling rate do not alias into the signal spectrum. By loweringthe bandwidth of the low-pass filter, a greater signal-to-noise ratio isprovided as long as signals of interest are not removed or reduced.Signals of interest provided by the probe 20 or representing a singleelement 24 are provided in a 2-15 MHz frequency range. The low-passfilter 40 is programmed with a 6 dB down or other cutoff frequency of 30MHz, 15 MHz less or other frequency. The bandwidth may be programmed asa function of the type of imaging or type of probe 20 used. Formultiplexed signals, such as time division multiplex information, thebandwidth is greater to pass multiplexed signals while minimizinginter-symbol interference. For example, the bandwidth is 30 MHz orgreater, such as 50 or 100 MHz, to provide a Nyquist channel shape or alinear-phase low-pass filter with the following magnitude responsesymmetry: |H(f)|=1−|H(Fsample−f)|, for 0<f<Fsample, where Fsample is themultiplexed sample rate (e.g. 96 MHz). In practice, H(f) is anapproximation to a Nyquist channel and errors are corrected via thedigital equalizer 46.

The buffer 42 comprises an amplifier or other analog components forbuffering signals input to the analog-to-digital converter 44. Thebuffer 42 provides the same characteristics regardless of the type ofdata or data format used, but may provide programmable characteristicsthat differ as a function of data format. For example, faster slew ratemay be required from 42 for multiplexed data. A programmable slew ratelimit could be used to conserve power in non-multiplexed modes.

The analog-to-digital converter 44 samples the analog signals andoutputs digital representations in any one of various now known or laterdeveloped codes. For data representing a single element 24, theanalog-to-digital converter 44 samples the data in response to a clockinput but without reference to other timing information. For timedivision multiplex data, the analog-to-digital converter clock input issynchronized with the multiplexer 26. The synchronization allows properseparation of signals from each of the different elements 24 withminimized cross signal interface.

The digitized samples are provided to an adaptive digital equalizer 46.The digital equalizer 46 comprises a programmable finite impulseresponse filter, such as implemented using a shift register 54,multipliers 56 and a summer 58. In alternative embodiments, a processoror other device is used to implement the equalizer 46. The digitalequalizer 46 filters time division multiplex information to removeinter-symbol interference. The filter coefficients applied to themultipliers 56 are based on a transfer function or generation ofinter-symbol interference from the element 24 through various stages orcomponents of the receive circuit 14 that operate on the analog signal.In one embodiment, the filter coefficients are programmable to allow foradaptations or variations in the transfer function. The coefficients areselected in response to a test signal or other data processingaccounting for detected differences in the transfer function, such ascaused by different probes 18, different processing characteristics ofanalog components of the receive circuit 14 or changes due to time andtemperature. For signals representing a single element 24 or signalsfree of multiplexing, the digital equalizer 46 passes the signals, suchas providing no delay in a single tap with a multiplier coefficient ofone.

The demultiplexer 48 comprises a digital demultiplexer, such as anetwork of switches for separating signals from various time slots in aframe of time division multiplex information. The demultiplexer 48operates as a conditional demultiplexer. The receive signals aredigitally demultiplexed. For example, the demultiplexer outputs signalsfrom different elements 24 on different outputs for beam formation andother image processing by the image processor 16. For conventionalsignals or signals free of multiplexing, the demultiplexer 48 passes theinformation to the image processor 16 for beam formation.

The optional analysis processor 50 comprises a digital signal processor,a general processor, an application specific integrated circuit, analogcomponents, digital components and combinations thereof forsynchronizing the analog-to-digital converter 44 with the multiplexer 26or selecting coefficients for the digital equalizer 46. The analysisprocessor 50 operates on a test signal. The probe control 28 causes themultiplexer 26 to transmit a known or predetermined digital or analogtest signal through the cable 22 and receive circuit 14 to the analysisprocessor 50.

The test signal is transmitted as part of a calibration function, suchas in response to user input or connection of the probe 18 to theconnector 32. The base unit 12 commands or the probe control 28automatically generate the test signals. In alternative embodiments,test signals are transmitted periodically. For example, a test signal istransmitted in a preamble or header for each frame of time divisionmultiplexed information. One or both of synchronization and adaptiveequalization are provided in response to periodic transmission of thetest signals. For stability, some phase sensitive acquisition sequences,such as acquisition for Doppler processing, minimize or do not provideany adaptation or changes in phasing through synchronization or theequalization.

One or both of multiplexing or processing of the receive signals isadapted in response to the analysis of the test signal. For example, theoperation of the multiplexer 26 is adapted to the operation of theanalog-to-digital converter 44 by synchronizing clock signals. Theanalysis processor 50 selects a selectable delay 52 for phasing theclock signal provided to the multiplexer 26 in reference to the analogdigital converter 44. Fixed delays in clocking circuitry, variabledelays due to clock signal path lengths, multiplexer circuit delays,multiplex signal path length, group delays and amplifiers anddigitization of delays cause misalignment, resulting in mixing signalsfrom different elements 24 by the analog-to-digital converter 44. Thesemisalignments may vary as a function of the probe 18, the receivecircuit configuration, time, temperature and processes. The analysisprocessor 50 determines the beginning of each frame by detecting a knownpattern or the test signal. Using the selectable delay 52, the phase ofthe clocking signals applied to the analog-to-digital converter 44 andthe multiplexer 26 are synchronized. In alternative embodiments, theanalog-to-digital converter clock signal is phased relative to the clocksignal provided to the multiplexer 26, or a group or subgroup of receivecircuits 14 are used to determine the phase of a clock signal common tomore than one multiplexer 26 relative to another clock signal common tomore than one analog-to-digital converter 44. The adaptive clockadjustments simplify the multiplexing control circuitry and interfacebetween the receive circuit 14 and the probe 18. One clock line or cable22 is provided without additional and separate phasing information. Inalternative embodiments, separate clock and phasing signals are providedto the probe controls 28.

In one embodiment, the processing by the receive circuit 14 is alteredor adaptive as a function of the test signal by the analysis processor52. For example, the analysis processor 50 selects coefficients from alookup table or calculates coefficients for use by the digital equalizer46. The digital equalizer provides symbol alignment or removal ofinter-symbol interference. The analysis processor 50 compares a known orstored test signal to the received test signal. Differences between thereceived test signal and the stored test signal are used to selectcoefficients. The coefficients are selected so that the receive signalsare undistorted or inter-symbol interference removed or diminished. Inalternative embodiments, results from more than one analysis processor50 are used to select coefficients for use by the digital equalizer 46.

In one embodiment, the receive circuit 14 includes a transmit receiveswitch. In alternative embodiments discussed below, no transmit andreceive switch is provided.

FIG. 2 represents a flow chart of one embodiment of operation of thesystem 10 of FIG. 1. In act 70, one of various possible probes 18, 20are connected with a base unit 12. One of the probes 18, 20 is selectedand attached to the connector 32. For example, a user desiresthree-dimensional cardiac imaging, so a two-dimensional array ofelements in the probe 18 associated with time division multiplexing isconnected.

For probes associated with multiplexing, a test signal is transmitted inact 72. Multiplexing or processing are adapted in response to the testsignal. For data free of multiplexing, act 72 is optional or notprovided. The test signal is transmitted in response to connection ofthe probe 18, response to control signals from the receive circuitry 14,in response to user input, automatically, or periodically. For example,a test signal is transmitted as part of an initial calibration processor is transmitted periodically in the header of first time slot or otherslot of each frame of time division multiplex information. The receivedtest signal is compared to an expected test signal. In response tocomparison, equalization coefficients or other processing of the receivecircuit is adapted or altered. Additionally or alternatively, the timingof the test signal is identified and selectable delays determined forsynchronizing the analog-to-digital converter 44 with the multiplexer26.

In act 74, the receive circuitry 14 is configured to have differentcharacteristics as a function of the type of probe or format of the datareceived from the probe 18, 20 connected with the receive circuitry 14.Where the data format corresponds to multiple elements, such as timedivision multiplexed data, the information is processed in response todifferent impedance, gain, filtering, equalization, analog to digitalconversion or other processes than for data associated with a singleelement or free of intervening circuitry in the probe 20. Any one orcombination of two or more of the various characteristics may be alteredas a function of the data format. Additional or differentcharacteristics may also or alternatively be altered. Act 74 may beperformed before or after act 72.

The analog information is then digitized. For time division multiplexinformation, the analog-to-digital converter 44 is synchronized with themultiplexed information. The multiplexed information is thendemultiplexed for beamformation and other imaging processes.

Transmit and Receive Isolation:

A transducer element 24 may be used to isolate the transmit channel fromthe receive channel in either of the probes 18, 20 discussed above oranother probe for use with different receive circuits. While useful forsingle element transducers, linear arrays, or arrays with limited or notransmit or receive circuitry within the probe, using a transducerelement 24 to isolate the transmit and receive channels is particularlyuseful for multi-dimensional transducer arrays with at least part oftransmit, and/or receive circuitry incorporated within the probe, suchas discussed above for the time division multiplexing probe 18. A fullypopulated multi-dimensional transducer array requires a large number oftransmit and receive channels. By placing transmit or receive circuitrywithin the probe and providing multiplexing, the number of cables 22 orchannels from the probe 18 to the base unit 12 are minimized. However,the transmit and receive circuitry then coexists in a small space,making isolation of the receive circuits from the high voltages of thetransmit circuits difficult. High voltage switches, such as switchesable to withstand 200 volts of reverse voltage, are difficult tointegrate with other receive circuits, such as a multiplexer. Highvoltage transmit and receive switching is replaced with the transducerelement for isolating the transmit channel from the receive channel.

FIG. 3 shows a transducer element 24 isolating or separating a transmitpath 62 and a receive path 64. Direct connection between the transmitpath 62 and the receive path 64 is avoided. The element 24 isolates thepaths 62, 64 to allow high voltage transmission without subjecting thereceive path 64 to the high voltage. High voltage devices are providedas part of the transmit path 62 but not as part of the receive path 64in one embodiment. In alternative embodiments, high voltage devices areprovided on the receive path 64.

The element 24 comprises one of a plurality of elements in amulti-dimensional or linear array. 1.5 dimensional and 2-dimensionalarrays may be represented as multi-dimensional arrays of a N×M grid ofelements where both N and M are greater than 1. For multi-dimensionalarrays, the elements may be small and have a high impedance as comparedto elements 24 of a linear array. Parasitic loading associated with acable 22 is also absent or reduced for use with a multiplexer and theprobe 18. A smaller transmit pulser and very low power receivepreamplifier may be used given the high element impedance than for alower impedance.

The element 24 includes two electrodes 80 and 82. The electrodes 80 and82 are on opposite of the element 24, such as being on a top and bottomof the element on a range dimension. The electrode 80 is free of anelectrical connection with the electrode 82. Separate signal tracescomprise or connect with each of the electrodes 80 and 82. Each element24 is associated with two or more separate signal traces for associatedseparate electrodes 80, 82. In alternative embodiments, two or moreelectrodes share a same signal trace. One electrode 80 connects to thetransmit path 62, and the other electrode 82 connects to the receivepath 64. The element 24 is free of an electrical connection directly toground, such as provided by an electrode connected directly to ground.

The transmit path 62 connects with the electrode 80 for applying atransmit waveform to the element 24. The transmit path 62 comprises atleast one signal trace to element 24 within the probe 18. In otherembodiments, additional transmit circuitry, such as a waveform generator84, a switch driver 87, and a controller 88 are incorporated within thetransmit path 62 and within the probe 18. In alternative embodiments,the controller 88, the driver 87, the waveform generator 84 orcombinations thereof are positioned external to the probe 18, such aswithin the base unit 12.

The waveform generator 84 comprises one or more high voltagetransistors, such as FET transistors, for generating unipolar, bipolaror sinusoidal waveforms. One embodiment of a transmit waveform generator84 for generating a unipolar waveform is shown in FIG. 4. Two highvoltage transistors 86, such as CMOS FET transistors with at least awithstand of 200 volts connect in series between a voltage source andground. In one embodiment, one transistor comprises a PFET, and theother transistor comprises an NFET. The transistors 86 provide highvoltage and ground driving of a unipolar waveform at the electrode 80.Since the transmit waveform generator 84 comprises a switch mode device,power dissipation is minimal. This circuitry for each element 24 usesabout 0.2 millimeters² of die area. For a 2-dimensional array of 1,536elements, about 307 millimeter² of die area is used. Other integrationformats may be provided, such as providing groups of high voltage FETtransistors in smaller application specific integrated circuits. Inalternative embodiments, other devices, such as digital-to-analogconverters, are used for waveform generation.

FIG. 5 shows a network of transistors 86 for generating a bipolarwaveform. Four transistors 86 allow generation of a bipolar waveformending with a positive voltage, negative voltage, or zero voltage. Threetransistors 86 may be used if the bipolar waveform is capable of endingat only one polarity, such as a positive voltage. Of the transistors, Q1and Q2 of FIGS. 4 and 5 may have an integral reverse diode from thedrain to the source, but transistors Q3 and Q4 avoid the reverse diodeconfiguration to avoid conducting through the diodes. Otherconfigurations and networks of transistors 86 may be used.

Each of the transistors 86 connects to a reference voltage, such as apositive voltage, a negative voltage or ground. As shown in FIG. 4, onetransistor 86 connects to ground and the other transistor 86 connects toa positive or negative voltage. As shown in FIG. 5, two transistors 86connect to ground, one transistor connects to a positive voltage, andanother transistor connects to a negative voltage.

The driver 87 comprises a transistor or FET driver for controllingoperation of the waveform generator 84. In alternative embodiments,other drivers may be used. The driver 87 is integrated as part of anapplication specific integrated circuit, but may have separate devicesor comprise a general processor. The driver 87 is operable to providevoltage changes for operating the transistors 86. For example, thetransistor Q2 of FIG. 4 is controlled by application of a 10 volt or 0volt signal from the driver 87. The transistor Q1 is controlled byapplication of a 200 volt or 190 volt signal from the driver 87.

The controller 88 comprises a general processor, analog components,digital components, application specific integrated circuit, orcombinations thereof for controlling one or more drivers 87 associatedwith one or more elements 24. In one embodiment, the controller 88 isintegrated on the same application specific integrated circuit as thedriver 87, but may be a separate device. The controller 88 outputsbinary signals to control the operation of the driver 87 and waveformgenerator 84. The controller 88 in one embodiment extrapolates orselects transmit configurations or waveform parameters for an entirearray or sub-array based on simple control signals provided fromexternal to the probe 18. In alternative embodiments, the controller 88is located external to the probe.

The receive path 64 comprises at least a single signal trace connectedwith the electrode 82 on an opposite side of the element 24 from thetransmit path 62. In other embodiments, the receive path 64 includes oneor more of diodes 90, 92, preamplifier 94 and a multiplexer 96.Additional, different or fewer circuits may be provided as part of thereceive path 64, such as a filter. The electronics may not contain anexplicit filter in the probe where the transducer element itself may besufficient and/or the natural low-pass response of the amplifier issufficient to filter the receive signal. The receive path is includedwithin the probe 18 with the element 24. In alternative embodiments, amultiplexer is not provided and the preamplifier 94 is provided in abase unit 12 separate from the probe 18 or in the probe 18. A cable 22connects the receive path 64 to the base unit 12.

The diodes 90 and 92 comprises Schottky diodes or other high current,low voltage diode devices. In one embodiment, the diodes 90 and 92 arefree of quiescent power dissipation. Each of the diodes 90 and 92connects to ground with an opposite or different polarity. The diodes 90and 92 comprise a diode clamp to limit voltage swings on the receivepath 64 at the electrode 82. For example, the diodes 90 and 92 limitvoltage transitions to between plus or minus 0.2 to 0.7 volts. Inalternative embodiments, transistors or other devices are used forlimiting the voltage at the electrode 82.

In one embodiment, the diodes 90 and 92 are integrated in an applicationspecific integrated circuit with the preamplifier and multiplexercircuits 94 and 96. Other integration formats may be provided, such asproviding discrete diode arrays and separate preamplifier/multiplexercircuits in smaller application specific integrated circuits.

The preamplifier 94 comprises one or more transistors for amplifying asignal from the electrode 82. For example, a differential BJT pair withcurrent outputs are provided using a 7 volt BiCMOS process or othertransistor process. Using 20 low μA per channel with a 5 volt supplyallows a consumption of 0.1 milliwatts per channel. Other preamplifierswith different power consumptions and associated components andcharacteristics may be used. The preamplifier 94 may alternatively oradditionally include a time or depth gain control amplifier or a filter.For a time gain control amplifier integrated within the probe 18, a lowpower device for providing some but not all of the time gaincompensation may be used. In alternative embodiments, a larger, morepower consuming variable amplifier is provided.

The multiplexer 96 comprises a network of switches, such as transistorsand analog sample and hold circuits for multiplexing the signals of aplurality of transmit paths 64 onto one cable 22. For example, themultiplexer 96 comprises an 8 to 1 multiplexer for multiplexing signalsfrom 8 different elements 24 into one frame of analog information. Inone embodiment, the multiplexer 96 is operable to provide 12 MSPS foreach receive path 64 for a total of 96 MSPS for 8 receive paths 64. Thecircuitry of the receive path 64 is free of high voltage devices and maybe integrated into one application specific integrated circuit or othergeneral circuit in a small space within the probe 18.

Connecting the transmit and receive path 62 and 64 to oppositeelectrodes 80 and 82, respectively isolates the high voltages and highvoltage devices of the transmit path 62 from the low voltage devices ofthe receive path 64. FIG. 6 shows a flow chart of one embodiment fortransmitting and receiving using the element 24 of FIG. 3. In act 100, ahigh voltage transmit waveform is provided to the transducer element 24,and the voltage in the receive path 64 is limited in act 102.Subsequently, the voltage on the transmit path 62 is limited in act 106and echo signals are received on the receive path 64 in act 104.

The transmit and receive operation of the element 24 is free of switchesto select between the transmit and receive path 62 and 64. In responseto control signals from the controller 88, the driver circuit 86 causesthe waveform generator 84 to generate a high voltage (e.g., 200 volt)transmit waveform in act 100. Where the waveform generator 84 ispositioned within the probe 18, the transmit waveform is generatedwithin the probe 18. The transmit waveform is applied to one electrode80 of the element 24. The voltage of the other electrode is limited,effectively acting as a ground or D.C. reference, in act 102. The diodes90 and 92 clamp the voltage of the receive path 64 connected to theelectrode 82 to within a small voltage range as compared to the highvoltage of the transmit waveform. In response, the element 24 generatesan acoustic signal due to the potential difference across the electrodes80 and 82. The element 24 also isolates the transmit path 62 from thereceive path 64, preventing damage to receive circuitry without highvoltage switching.

For a subsequent receive operation of act 104, the voltage at thetransmit path 62 is limited. In one embodiment, a transistor 86 of thewaveform generator 84 connects a ground or reference voltage to theelectrode 80. For example, Q2 of the waveform generator 84 shown in FIG.4 is switched “on” to ground the electrode 80. In an alternativeembodiment, another reference voltage, such as a positive voltageapplied through Q1 is connected to the electrode 80 to limit the voltageswing or change of the electrode 80. While the voltage of the transmitpath and associated electrode is limited in act 106, electrical signalsare generated at the electrode 82 in response to acoustic echo signalsreceived by the element 24 in act 104. Since the electrical signalsreceived are small, such as less than 0.2 volts, the diodes 90 and 92avoid introducing noise within or clipping the receive signal. Thereceive signal is amplified, filtered, multiplexed, or otherwiseprocessed for transmission over the cable 22 to the base unit 12. Forexample, the amplifier 94 preamplifies the signals and adjusts the gainof the electrical signals as a function of time. The multiplexer 96multiplexes the electrical signals with other electrical signalsresponsive to different transducer elements 24. The same process isrepeated for receive channels 64 associated with other elements 24. Thetransmit and receive operations are performed free of selecting betweentransmit and receive paths for connection with an electrode. Each of thetransmit and receive paths 62 and 64 act to ground or otherwise maintainan electrode 80, 82 at a reference voltage during reception andtransmission, respectively.

Using the waveform generator 84 shown in FIG. 4, unipolar waveforms maybe generated ending either with zero voltage or a positive voltage. Theunipolar waveform generator 84 is capable of ending on a positive orzero voltage state without damage to the circuit. An alternativeembodiment would allow unipolar waveform generation between zero and anegative voltage by swapping the NMOS and PMOS devices and using anegative power supply. In either case a low impedance condition isprovided whether the unipolar transmit waveform ends at a 0 voltage orother voltage.

FIG. 7 shows two mirror symmetric unipolar waveforms 108 and 110. Thefirst unipolar waveform 108 begins at a low state or zero voltage level,includes a positive voltage pulse, returns to a 0 voltage level and thenends at a high state or positive voltage level. The subsequent unipolarwaveform 110 begins at a high state or positive voltage and ends at alow state or zero voltage. Since one waveform begins at the highervoltage and ends at the lower voltage and the other waveform 108 beginsat the lower voltage and ends at the higher voltage with the same numberof cycles, the two waveforms sum, to substantially a zero value.Substantially accounts for differences in rise and fall path times ofthe transistors 86 and other differences in performance using transmitwaveforms beginning at different voltages. In alternative embodiments,the high state is zero volts and the low state is a negative voltage.

The mirror symmetric capability of the unipolar waveform generator 84allows for tissue harmonic or other harmonic imaging using phaseinversion with unipolar transmit waveforms. As acoustic energyresponsive to the transmit waves propagates and scatters within tissue,energy at second harmonics or other harmonics of the fundamentaltransmit frequency is generated. The receive signals responsive to eachof the unipolar waveforms include information at the fundamentalfrequencies as well as the harmonic frequencies. When the receivesignals responsive to the phase inverted transmit unipolar waveforms arecombined or added, information at the fundamental frequencies cancels,leaving information at harmonic frequencies.

Harmonic imaging in response to phase inversion of transmit waveforms isprovided using simple unipolar waveforms. The transistors 86 used forgenerating the unipolar waveform are designed to avoid rise time andfall time mismatches, minimizing the amount of harmonic informationintroduced by the waveform generator 84. The material of the element 24has a high poling voltage in one embodiment to minimize differences inoperation or receive mismatches due to initial generation at twodifferent DC bias points (e.g. 0 and +V). Transmission of a phaseinverted unipolar pulses may be used with systems having a transmitchannel within the base unit or within the probe, and with systems usingtransmit and receive switching.

Multi-Dimensional Transducer:

Various transducers can be used with any of the transmit and receivepaths, probes and receive circuits discussed above. Some suchmulti-dimensional transducer arrays for fully sampled use with timedivision multiplexing and element based isolation of transmit andreceive paths is shown in FIGS. 8-11. Time division multiplexing reducesthe channel count or number of cables 22 without limiting thebeamforming performed by the base unit 12. Separate signal traces orconnection of opposite electrodes 80 and 82 to transmit and receive pathallows integration of transmit and receive circuitry in the probe 18without power consuming transmit and receive switching. Various aspectsof the multi-dimensional transducer may be used independent of otheraspects of the embodiments described herein, such as using a particularelement spacing without time division multiplexing or other integrationof circuitry within the probe 18.

FIG. 8 shows a 2-dimensional array 200 of elements 24. The elements 24are spaced in a grid along the elevation and azimuth dimensions. Adifferent or same number of elements 24 may be provided along theelevation dimension than along the azimuth dimension. A plurality ofelements 24 are provided in columns 204 along the azimuth dimension. Theelements 24 have a pitch or spacing along the azimuth dimension. In oneembodiment, a ½ wavelength pitch is used. From the center of one elementto the center of an adjacent element 24 along the azimuth dimension, adistance of ½ of a wavelength is provided. For example, in an arraydesigned for operation at 2.5 MHz, the pitch is 300 micrometers. Otherspacings may be used.

The elements 24 are provided in rows 202 along the elevation dimension.The pitch or spacing along the elevation dimension is greater than thepitch or spacing along the azimuth dimension. In one embodiment, thepitch along the azimuth dimension is ⅔ or less, such as ½, than thepitch along the elevation dimension. For the 2.5 MHz center frequencyarray example given above, the pitch in elevation is 600 micrometers orone wavelength. For large pitches, each individual element may besub-diced for proper operation or to maintain a desired ratio of thewidth to a thickness of the element 24. In the example provided above,the elements 24 are sub-diced along the elevation dimension, such asproviding a dicing cut extending through about 90 percent of PZTmaterial at the center of each array, but not sub-diced along theazimuth dimension. Other sub-dicing depths may be used.

FIG. 8 shows thirty-two elements 24. In alternative embodiments,different numbers of elements are provided, such as 1,536 elements in 64azimuthally spaced rows 202 and 24 elevational spaced columns 204, or2,048 elements in 64 azimuthally spaced rows 202 and 32 elevation spacedcolumns 204.

FIG. 9 shows a probe 18 integrating the array 200. The probe 18 includesthe array 200, flexible circuit materials or signal traces 206, 208, aplurality of circuit boards 210, a capacitor 212 and a bundle of cables22. These components are housed within a plastic or other ergonomicallyshaped probe cover or housing. Different, fewer or additional componentsmay be included in the probe 18.

The flexible circuits 206, 208 comprise Kapton or other flexible, thin,electrical insulating material with deposited signal traces on one ortwo sides. Flexible circuit is used herein to describe any flexible ornon-rigid material with one or more electrical conductors. In oneembodiment, the flexible circuit material is 50 μm thick. Separateflexible circuit materials 206 and 208 are provided for separatetransmit and receive paths. For example, one flexible circuit 206provides electrodes and traces from one side of the elements 24 of thearray 200, and the other flexible circuit 208 comprises electrodes andtraces from an opposite or different side of the elements 204 of thearray 200.

FIG. 10 shows an elevation cross-section of the array 200 and theassociated connections of the two flexible circuits 206 and 208. Thearray 200 is subdivided along the elevation dimension into four modules222. Additionally, the array 200 may be subdivided along the elevationdirection into different or fewer modules 222. For example, only one,two, three, or more modules may be used. Each module has an associatedpair of flexible circuits 206 and 208. Each module 222 includes aplurality of layers along the range dimension, such as a first matchinglayer 218, a first electrode layer on the top of the element 24 formedfrom the first flexible circuit 208, a second matching layer 216, anelement or piezoelectric (PZT) layer 214, a second electrode on a bottomside of the piezoelectric layer 214 formed from the second flexiblecircuit 208 and a backing material 220. Additional, different or fewerlayers may be provided in one, more or all of the modules 222. Forexample, only one or three or more matching layers 216, 218 are used, orboth matching layers 216 and 218 are on a top side of the top electrodeand flexible circuit 208.

The two different flexible circuits 208 and 206 are folded along one ortwo sides of the modules from the PZT material or layer 214 towards andalong the backing material 220. Separate signal traces are provided toeach of the elements 24 on both sides or top and bottom of the elements24. Separate signal traces are provided on the flexible circuit 206 foreach of the elements 24, and separate signal traces are provided on theflexible circuit 208 for each of the elements 24. Each of the elements24 independently connects with the separate signal traces on the top andbottom along the range dimension of the element 24. Separate signaltraces allow for element based isolation of the transmit and receivepaths. In alternative embodiments, a common ground connects with aplurality of elements 24.

The backing material 220 of each module 222 is separated from the otherbacking material 220 of another module 222 by two or four layers offlexible circuit 206, 208. The PZT layer 214 of one module 222 isseparated by one or two flexible circuit layers 208 from the PZT layer214 of another module 222. The width of the PZT layer 214 is greaterthan the width of the backing material 220 to account for the differentthicknesses due to the different number of flexible circuits 206, 208.By having a thin flexible circuit material, adverse acoustic effects areavoided by minimizing the separation between elements 24 of differentmodules 222.

Referring again to FIG. 9, the flexible circuits 208 and 206 are shownas having an increasing width away from the array of elements 200.Increasing the width allows for greater separation of the signal tracesfrom individual elements 24. The greater separation provides for lesscapacitive coupling between signal traces.

FIG. 9 shows a plurality of printed circuit boards 210, such as sevencircuit boards 210. In alternative embodiments, a single circuit board210, a different number of circuit boards 210 or no circuit board areprovided in the probe 18. In one embodiment, six circuit boards 210include transmit and receive circuitry, such as the probe integratedcircuitry discussed above. Each transmit and receive circuit board 210connects with one of six elevationally spaced modules 222. Inalternative embodiments, one circuit board 210 connects with elements 24in different modules 222, or elements 24 in a same module 222 connectwith different circuit boards 210. A seventh circuit board comprises acontrol logic circuit board. The control logic circuit board interfaceswith the base unit 12 for operating the transmit and receive circuitry.The printed circuit boards 210 and other components of the probe 18 aresized to fit within the handle of the probe 18. The probe 18 is designedfor ergonomic handling by a user, such as being less than four inches indiameter or providing a handheld grip.

In one embodiment, the circuit boards include one or more multiplexers.For example, a plurality of eight to one multiplexers are provided formultiplexing signals from the elements 24 onto 192 system channels orcables 22. In other embodiments, fewer or more multiplexers for use withfewer or more cables 22 or system channels are provided. For example, anarray 200 of 64 rows 202 and 32 columns 204 is provided withmultiplexers for transmitting time division multiplexed information on256 cables 22. Providing the multiplexer in the probe 18 with the array200, fewer cables 22 and associated system channels or signal lines areprovided than the number of elements 24 of the array 200. For example,the product of the number of elements along the elevation dimension andthe number of elements along the azimuth dimension is greater than thenumber of cables 22.

The circuit boards 210 connect with the flexible circuits 206 and 208using any now-known or later developed connectors or connections. Usingtwo or more separate signal traces for each element 24 provideselectrical connections for twice the number of elements 24. Theconnectors are attached to the flexible circuits 206, 208 prior to arrayfabrication. In one embodiment, a ball grid array (BGA) or other matrixof bumps or other structures for soldering to the traces on the flexiblecircuits 206 and 208 are provided. Small pitch matrix type BGAconnectors may be used. For example, the BGA connects the receive pathsignal traces to the multiplexer, and the multiplexer is then connectedto the printed circuit boards, reducing the number of connections to theprinted circuit boards. In another embodiment, transmit or receivecircuitry are deposited or otherwise formed on the flexible circuit,resulting in the need for fewer connections from the flexible circuits206, 208 to the printed circuit boards 210. In yet another embodiment, adirect attachment, such as wire bond jumping or other interconnections,is provided between the flexible circuit and the printed circuit boards.

FIGS. 11A and B represent acts in a process for manufacturing the array200. FIG. 11A shows three modules of elements 24. Each module 222includes at least two rows and two columns of elements 24 in an N by Marray. The PZT layer 214 of each module 222 and the associated flexiblecircuits 208, 206 are diced independently for each module 222. Thedicing includes one or both of dicing along the azimuth or elevationdimensions to form the elements 24. By dicing the electrodes or flexiblecircuits 208, 206 separately for each module 222, each module 222 may betested separately. Separate testing allows for disposal of a defectivemodule 222 before final assembly. For example, capacitants tests oracoustic tests are performed for each element 24 of each of the modules222.

Each of the separately diced modules 222 is formed as shown in FIG. 10.Any of various manufacturing processes may be used, and different ordersof assembly provided. In one embodiment, the first matching layer 216, aslab of piezoelectric layer 214 and a flexible circuit 206 positioned onthe bottom of the piezoelectric layer 214 are stacked on top of thebacking layer 220. Precision tooling with pins and associated holes ortemplates are used for aligning these layers. The bottom flexiblecircuit 206 has signal traces on both sides for connecting withdifferent elements 24. The aligned layers are then bonded or gluedtogether.

After bonding, the bottom layer of flexible circuit material 206 isfolded along the sides of the backing layer 220 below the layer ofpiezoelectric layer 214. The width of the backing layer 220 is narrowerthan the width of the piezoelectric layer 214 by about the width of oneor two layers of the flexible circuit 206. In one embodiment, theflexible circuit 206 is folded on two sides of the backing layer 220,but may be folded on just one side. The bottom flexible circuit 206 istightly bonded to the backing material by placing the partial module 222through a Teflon coated or other frame with bonding material or glue.Alternatively, the bottom flexible circuit 206 is bonded to the sides ofthe module 222 during a later act of bonding the top flexible circuit208.

The first matching layer 216 and piezoelectric layer 214 are diced alongthe azimuth dimension. For example, six major dicing kerfs are formedthat extend into, but not through, the flexible circuit material 206.Minor dicing kerfs may also be formed along the azimuth dimension. Theminor dicing kerfs extend about 90% into the piezoelectric layer 214.Other dicing depths may be used. Epoxy, silicone or other material isused to fill the diced kerfs. A kerf-filling material with a higheracoustic impedance may be used since only the piezoelectric layer 214and the first matching layer 216 are diced in azimuth. In alternativeembodiments, other layer are diced in azimuth and a lower acousticimpedance kerf-filling material is used. In alternative embodiments, nokerf-filling material is used.

The surface of the first matching layer 216 is ground or otherwiseprocessed to remove any excess kerf-filling material if necessary. Thetop flexible circuit 208 and the second matching layer 218 are alignedand bonded together using pins and holes or templates. The bonded topflexible circuit 208 is then bonded to bottom matching layer 216. Inalternative embodiments, the top flex 208 and top matching layer 218 arealigned and bonded to the bottom matching layer 216 on the module 222 asone operation associated with filling the kerfs.

The position of the top flex circuit 208 and associated signal tracesrelative to the bottom flex circuit 206 and associated signal traces iswithin a tolerance sufficient to allow separate signal traces for eachelement 24. For example, a tolerance of plus or minus 50 μm allows adicing area of 100 μm between each of the elements 24 along theelevation dimension without adversely cutting a signal trace. Othertolerances and distances are possible. The alignment is performed usingprecision-tooled pins and holes, template or optical alignment. Byproviding signal traces on flex circuits 206, 208 on both sides of themodule 222, less dense signal traces are provided, allowing largerdicing windows. In alternative embodiments, a greater density of signaltraces is provided and the flexible circuits 208 and 206 are provided onone side of the module 222.

The top flexible circuit 208 is folded along one or two sides of thepiezoelectric layer 214 and backing layer 220. The flexible circuit 208extends from the piezoelectric layer 214 towards the backing material220. Where signal traces are provided on a top side or outward facingside of the bottom flexible circuit 206 and on a bottom or inward facingside of the top flexible circuit 208, an insulation layer is addedbetween the two flexible circuits 206 and 208. For example, a 25 μm orother thickness of Teflon or electrically non-conductive material isapplied to one or both of the flexible circuit layers 206 prior toassembly or during assembly. The upper flexible circuit 208 is thenbonded to the sides of the modules 222 by passing through a frame with aTeflon coating or other coating. Both flexible circuits and theassociated electrodes are bonded to the module 222.

The module 222 is then diced in the elevation dimension, such as dicingto form 64 columns 204 of elements 24. The dicing extends through bothflexible circuits 206 and 208 and the piezoelectric layer 214 into thebacking layer 220. In one embodiment, no minor dicing kerfs areprovided, but minor dicing kerfs may be used. In one embodiment, the topflexible circuit 208 is examined through a microscope for opticallyaligning the dicing saw. The elevational dices in combination with theearlier azimuthal dices define the elements 24. The elevational dicesmay be provided for each of the modules 222 at a same time or atdifferent times. The dicing results in top and bottom separateelectrodes and associated signal traces for each of the elements 24without a grounding plane common to all of the elements. In alternativeembodiments, a grounding plane is used with only one separate signaltrace for each element 24.

The separately diced modules 222 are aligned as shown in FIG. 11B. Themodules 222 are positioned adjacent to each other along the elevation orazimuth dimension to form a larger array 200 of elements 24. Each of themodules 222 is separated from another module 222 by one or more of theflexible circuits 206, 208. In one embodiment, each of the modules 222represents 64 azimuthally-spaced rows 202 and four or sixelevationally-spaced columns 204 of elements 24. By aligning four or sixmodules 222 in the elevation and azimuth dimensions, a 64 by 24 grid ofelements 24 is provided. Other number of modules, sizes and number ofelements grids may be used with or without separation of modules 222 byflexible circuits 206, 208.

The top flexible circuit 208 has signal traces formed on a bottom sideso that the flexible circuit 208 electrically insulates the signaltraces of one module 222 from the signal traces of another module 222.In alternative embodiments, an insulator material, such as additionalKapton or other material, is positioned between the two modules 200 forelectrical isolation of the signal traces.

Prior to aligning, each of the modules 222 is pressed through aTeflon-covered frame or other frame with glue or other bonding material.The pressing tightly fits the flexible circuits 206 and 208 along thesides of the modules 222 for minimizing any separation between modules.

The modules 222 are positioned within a frame 224. The frame comprises agraphite material, another conductive material, or other non-conductivematerial. The four modules 222 either press-fit within the frame 224 orare positionable within the frame 224. When the modules 222 arepositioned within the frame 224, the spacing between the PZT layer 214of the modules 222 is 50-150 micrometers, but other spacing may be used.The spacing is the result of the flexible circuit material between thepiezoelectric layers 214 of each module 222. A 50-150 micrometer spacingis either 0-100 micrometers larger than a normal kerf width. Otherrelative widths may be used. Minimizing the separation between modules222 minimizes the beam width in the elevation dimension or the elevationpoint spread function. The frame 224 aligns the modules 222 in bothdimensions but may provide less tolerance within an azimuth dimension.Higher tolerance alignment may be provided through manual opticalalignment, pin and hole alignment or precise machining of the frame 224as a template.

After the modules 222 are aligned within the frame 224, the kerfs fromthe separate dicing are filled with silicone or other kerf-fillingmaterial. The kerf-filling material also acts to bond the modules 222 toeach other and the frame 224. In alternative embodiments, the kerfs ofthe modules 222 are filled prior to alignment. In alternativeembodiments, no kerf filling is used. A protective layer of lensmaterial or other focusing or non-focusing acoustically transparentmaterial is positioned over or around the array 200. For example, hightemperature or room temperature vulcanized silicon is formed over thearray 208. Where the array 200 is fully sampled, the additionalprotective layer provides for no focus or limited focus.

The flexible circuits 206, 208 and associated signal traces areconnected to the printed circuit boards or multiplexers. The output ofthe multiplexers are connected to cables 22. The cables electricallyconnect the elements 24 of the array 200 to the base unit 12.

In alternative embodiments, different multi-dimensional arrays areprovided with a multiplexer integrated within the probe 18 and/orisolation of transmit and receive paths by the transducer element 24.Multiplexing allows multiplexing of multiple channels onto a singlechannel, such as through time division multiplexing. The amount ofmultiplexing, the bandwidth desired, the center frequency, and the clockrate determine the amount of multiplexing used. For example, a systemwith a 40 MHz clock rate may use up to a 25 MHz center frequencytransducer assuming Nyquist sampling rate up to 1.6 times the centerfrequency. With multiplexing, the center frequency may be reduced toreduce the number of system channels or cables 22. In the example above,a 2:1 multiplexer allows use of up to a 12.5 MHz center frequencytransducer with a 120% bandwidth, but doubles the number of elements 24using one cable 22. A 3:1 multiplexer allows use of up to a 8.3 MHzcenter frequency transducer. 4:1 allows 6.3 MHz, 5:1 allows 5.0 MHz, 6:1allows 4.2 MHz, 7:1 allows 3.6 MHz and 8:1 allows 2.5 MHz. Higher clockrates allow either more multiplexing or higher center frequencytransducers.

Some multi-dimensional arrays provide a plurality of transducer elementsarranged with N elements along a first dimension where N is greater thanone and with M elements along a second dimension where M is greater thanone and not equal to N. For example, a multi-PZT layer linear array, a1.5D, I-beam, +-beam or other arrays of elements 24 have differentdistributions of elements 24. A probe houses the array 200 of elements24. A multiplexer within the probe and connected to at least two of theplurality of transducer elements 24 allows for a greater number ofelements 24 with a fewer number of system channels or cables 22connected to the base unit 12.

Multiplexing allows higher resolution use of 1.5 dimensional transducerarrays, such as arrays with two or more elevation rows of 96 elements 24in the azimuth dimension. For example, with 2:1 time domainmultiplexing, a 1.5D array with three or four rows of 96 elements uses192 system channels or cables 22 at up to 12.5 MHz. With 7:1multiplexing of 7 segments or rows of 96 elements 24, the array mayoperate at up to 3.6 MHz with 192 system channels or cables 22 in a 40MHz clock rate system.

A plano-concave transducer with isolated left and right elevationaperture spaced elements 24 may also benefit from multiplexing. Forexample, see the arrays described in U.S. Pat. No. 6,043,589, thedisclosure of which is incorporated herein by reference. Two or threesegmented arrays operate at a higher center frequency and/or with moreelements by multiplexing signals from one or more elements with signalsfrom another element.

Transducers configured as two or more separate or intersecting linear orcurved linear arrays may also benefit from multiplexing. A first lineararray is positioned along one dimension and a second linear array ispositioned along the second dimension or not parallel to the firstarray. For example, the various I-beam, +-beam or other arrays disclosedin U.S. Pat. No. 6,014,473, the disclosure of which is incorporatedherein by reference, use multiplexing to allow for a greater number ofelements with the same or fewer cables 22. In this example, one lineararray is used for imaging and one or more other orthogonal arraysprovide tracking information. By multiplexing, image resolution issacrificed less by using system channels or cables for tracking arrays.For example, one imaging and two tracking arrays each use 192 elements24 with 3:1 multiplexing to 192 cables 22. Other distributions ofelements 24 within the arrays may be used.

Bi-layer or multiple layer transducer arrays may also benefit frommultiplexing. Two or more layers of PZT within a linear or other arrayof elements 24 are used for harmonic imaging. One or more onedimensional arrays of elements 24 along the azimuth dimension havelayers of elements 24 or PZT along the range dimension. For example, thearrays disclosed in U.S. Pat. Nos. 6,673,016 (Ser. No. 10/076,688, filedFeb. 14, 2002) or 5,957,851 use multiple layers of elements 24 separatedby electrodes. Multiplexing allows for a greater number of separatelyaddressable PZT layers and/or elements 24. The relative phasing of onelayer to another layer provides for either fundamental or harmonicoperations.

A square grid of elements as a two-dimensional array or a single lineararray may also benefit from multiplexing. Multiplexing allows for moreelements with fewer system channels or cables 22. Multiplexing provideshigher resolution and/or faster scanning for two or three dimensionalimaging.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscan be made without departing from the scope of the invention. It istherefore intended that the foregoing detailed description be understoodas an illustration of the presently preferred embodiment of theinvention, and not as a definition of the invention. It is only thefollowing claims, including all equivalents, that are intended to definethe scope of this invention.

What is claimed is:
 1. A system for transmit and receive isolation forultrasound processing, the system comprising: a transducer elementhaving first and second electrodes; a transmit path connected to thefirst electrode; and a receive path connected to the second electrode,the receive path separate from the transmit path at the transducerelement.
 2. The system of claim 1 wherein the transmit path comprises awaveform generator, the waveform drive circuit positioned in a probe,the transducer element also positioned in the probe, and furthercomprising: a cable connectable between the probe and a base unit. 3.The system of claim 1 wherein the receive path comprises at least oneamplifier and a filter, the at least one amplifier and filter positionedin a probe, the transducer element also positioned in the probe, andfurther comprising: a cable connectable between the probe and a baseunit.
 4. The system of claim 1 wherein the transmit path comprises awaveform generator and the receive path comprises a time gain controlcircuit, the waveform generator, time gain control circuit andtransducer element in a probe, the probe separate from an imaging baseunit.
 5. The system of claim 1 wherein the receive path comprises amultiplexer.
 6. The system of claim 1 wherein the receive path comprisesat least two diodes electrically connected with the second electrode. 7.The system of claim 6 wherein the at least two diodes electricallyconnect between the second electrode and a ground, the two diodescomprising a diode clamp.
 8. The system of claim 1 further comprising aplurality of additional transducer elements, the transducer element andadditional transducer elements arranged in an N×M grid where N and M areboth greater than one.
 9. The system of claim 1 wherein the transmitpath comprises at least one transistor electrically connectable betweenthe first electrode and a reference potential.
 10. The system of claim 1wherein the transmit path is operable to connect the first electrode toground during receive operation of the transducer element and thereceive path is operable to limit a voltage on the second electrodeduring transmit operation of the transducer element.
 11. The system ofclaim 1 wherein the transmit path includes at least one high voltagecomponent and the receive path is free of high voltage components. 12.The system of claim 1 wherein the transmit and receive paths are free ofany switch operable to select between the transmit path and the receivepath.
 13. The system of claim 1 wherein all electrodes of the transducerelement are free of a direct connection to ground.
 14. A method forultrasound isolation of transmit and receive events, the methodcomprising: (a) applying a transmit waveform to a first electrode of atransducer element; (b) limiting the voltage at a second electrodeduring (a); (c) receiving electrical signals on the second electrode ofthe transducer element, the second electrode different than the firstelectrode; and (d) limiting the voltage at the first electrode during(c).
 15. The method of claim 14 further comprising: (e) generating thetransmit waveform in a probe where the transducer element is also in theprobe.
 16. The method of claim 14 further comprising: (e) filtering theelectrical signals with a filter, the filter in a probe wherein thetransducer element is also in the probe.
 17. The method of claim 14wherein (a) comprises driving the transducer element with drivercircuits in a probe separate from an imaging base unit, the probeincluding the transducer element, and further comprising: (e) adjustinggain of the electrical signals as a function of time with a time gaincontrol circuit, the time gain control circuit in the probe.
 18. Themethod of claim 14 further comprising: (e) multiplexing the electricalsignals with signals responsive to different transducer elements. 19.The method of claim 14 wherein (d) comprises clamping the secondelectrode with at least two diodes electrically connected with thesecond electrode.
 20. The method of claim 14 wherein (b) compriseselectrically connecting the first electrode with a reference potential.21. The method of claim 14 further comprising: (e) performing (a)-(d)for a plurality of transducer elements arranged as a two-dimensionalarray.
 22. The method of claim 14 wherein (a) and (d) are performed withat least one high voltage component and (b) and (c) are performed freeof high voltage components.
 23. The method of claim 14 wherein (a)-(d)are performed free of selecting between a transmit path and a receivepath.
 24. The method of claim 14 wherein (a) comprises applying aunipolar waveform, a beginning of the unipolar waveform being a firststate and the ending of the unipolar waveform being a second statedifferent than the first state, the first and second states comprisingdifferent ones of a high state and a low state.
 25. A method fortransmitting acoustic energy with phase inversion, the methodcomprising: (a) generating a first unipolar transmit waveform having ahigh state and a low state; (b) generating a second unipolar transmitwaveform having the high state and the low state; (c) beginning thefirst unipolar transmit waveform in the low state; and (d) beginning thesecond unipolar transmit waveform in the high state.
 26. The method ofclaim 25 further comprising: (e) ending the first unipolar transmitwaveform in the high state; and (f) ending the second unipolar transmitwaveform in the low state.
 27. The method of claim 25 wherein (a)-(d)comprise generating the first and second unipolar transmit waveformssuch that a sum of the transmit waveforms as applied to a transducerelement is substantially zero.
 28. The method of claim 25 wherein (c)comprises beginning the first unipolar transmit waveform at a zerovoltage and (d) comprises beginning the second unipolar transmitwaveform at a positive voltage.